Coating systems for automobiles normally include a multiplicity of coatings applied to a steel substrate. Typically, the steel is treated with a rust-proofing phosphate layer, then a cathodic electro-coat primer for additional corrosion protection is applied. A primer-surfacer (also known as a chip resistant primer, primer, or primer filler) is used next to smooth the surface for top-coating and also to provide stone chipping resistance to the coating system during the normal course of driving. Then a top-coat system is applied, sometimes as a single colored coat, more often now as a basecoat with solid color or flake pigments followed by a transparent protective clear coat, to protect and preserve the attractive aesthetic qualities of the finish on the vehicle even on prolonged exposure to the environment or weathering.
Coating film formation of the basecoat and the clearcoat is normally achieved by wet-on-wet application, which is to say that the clearcoat is applied to the basecoat without baking the basecoat prior to clearcoat application (although the basecoat may be flash dried for a short period of time at room temperature prior to clearcoat application), and then subsequently baking the basecoat and clearcoat at the same time to form a dried and cured finish. In the conventional method for forming the multi-layer coating film, the underlying primer surfacer layer, however, is baked before being top-coated with basecoat and clearcoat. Historically, baked primers have been used not only to provide a smooth surface on which to apply the topcoat, but also to prevent interfacial bleeding or intermixing with the overlying basecoat and avoid disrupting the appearance of the overall topcoat finish. Resistance to intermixing (sometimes referred to as “strike-in” resistance) is especially important for the appearance of glamour metallic finishes which are very popular nowadays on automobiles and trucks. Any disturbance of the metallic pigment flake orientation in metallic basecoats after application over the primer-surfacer will detract from the metallic effect of the finish. Therefore, care must be taken to ensure that the metal pigment flakes are not disturbed after painting.
In recent years, it has also been strongly desired to reduce the environmental load or impact of automotive assembly plants by reducing VOC (volatile organic compounds) emissions and CO2 (carbon dioxide) emissions generated from operating painting booths and baking ovens. This has led to use of lower solvent content in the paint and the development of three-layer wet paint systems which make it possible to apply a primer surfacer, basecoat, and clearcoat wet-on-wet continuously before they are cured all at once in a single bake. With this simplified application process, it is possible to eliminate the separate primer painting booth and primer oven, which also results in substantial cost savings to the automobile manufacturers. The technical hurdles of this process simplification, however, have been significant. For instance, interfacial bleeding and aesthetic appearance, such as surface smoothness, as well as film properties such as chip resistance, are still significant concerns.
Attempts have been made to address the forgoing problems by modifying the formulation of the primer coating material. For example, some prior art methods for forming a multilayer automotive coating film use a three layer wet paint process (also referred to as a “3 wet” or a “3-coat-1-bake” process) wherein a standard polyester-melamine primer coating is formulated to also contain acrylic polymer particles, namely in the form of internally crosslinked nonaqueous dispersion (NAD) polymers or internally crosslinked microgel particles. These particles are intended to raise the viscosity and solubility parameter between the primer surfacer and the base coating to prevent intermixing at the interface between the coated layers. However, use of such particle-filled systems also suffers from some drawbacks.
For example, the microparticles also tend to create voids in the surface of the wet primer where the basecoat can still flow in and intermix, resulting in defects in the aesthetic appearance such as loss of smoothness, gloss, head on brightness, and/or metallic effect. Sagging of these coatings, especially on vertical panels, such as doors, fenders, rocker panels, etc., is also a problem. These particle-filled systems are also not able to maintain dry film builds at normal commercial levels. Film builds must therefore be reduced to allow the NAD or microgel particle to migrate to the interface. Yet, thin films are an impediment as they tend to subject the underlying corrosion-protective electrocoated primer layer to excessive UV light transmission and deterioration. Thin films or thin film regions are also inadequate for mechanical properties and visual appearance of the overall finish.
Accordingly, it is desirable to provide a more effective way to improve the aesthetic appearance and surface smoothness of primer surfacer, basecoat, and clearcoat layers when applied in a wet on wet on wet (i.e., a “3 wet”) manner and make it possible to eliminate the primer baking process and reduce the environmental impact of the coating system, while also maintaining film builds, the overall appearance such as high gloss and distinctness of image and film properties of the coating system.